Aleurone layer

In seeds that possess a solitary cotyledon and endosperm, the endosperm is surrounded by a specialized, slender layer composed of a single cell, which is referred to as the aleurone layer. The endosperm cells responsible for the storage of starch are relatively large and exhibit irregular shapes, containing deposits of starch granules. In contrast, the aleurone cells are cuboidal in shape and enclose aleurone grains.

The principal function of the aleurone layer is to furnish nourishment to the embryo enclosed within the seed. This layer acts as a protein-rich covering formed by the outermost cells of the endosperm. It actively releases specific enzymes that contribute to the facilitation of seed germination.

The aleurone layer also has the potential to express diverse proteins that provide protection against pathogens, including the protein PR-4. Moreover, the aleurone layer serves as a highly advantageous dietary element in many grain bran products. Additionally, within the aleurone tissue, there are abundant protein-storing vacuoles known as protein bodies.

In contrast to the endosperm cells responsible for starch storage, the aleurone cells maintain their viability within mature grains. Nonetheless, they undergo programmed cell death (PCD) when germination is initiated or when isolated aleurone layers or protoplasts are exposed to gibberellic acid (GA).

This layer covers the entirety of the starchy endosperm and the germ, experiencing interruptions only at the kernel's tip in the form of the hilar layer. The aleurone layer is thinnest over the germ. Comprising parenchyma cells that are alive, the aleurone tissue consists of three to four cell layers in barley and rice, and just one cell layer in wheat, oat, and maize.

These cells possess substantial walls that are rich in arabinans and arabinoxylans, while having limited cellulose content. They are connected through numerous plasmodesmata.
Aleurone layer